This invention relates generally to refrigeration compressors, and more particularly to hermetic refrigeration compressors of the fractional horsepower type used in household appliances such as refrigerators and freezers.
There has been a continuing need for increased energy efficiency in these appliances and one area where much improvement has been obtained is in the hermetic compressor. While much of the improvement has been in the electric motor portion of the compressor, there still remains further room in the area of volumetric and compression efficiency of the reciprocating piston compressor.
One of the factors affecting the volumetric efficiency of these compressors is the clearance or re-expansion volume of the pumping cylinder, which is defined as the volume of space within the pumping cylinder when the piston is at top center or the end of its pumping stroke. This space consists essentially of the space between the piston face and the valve plate on which the suction and discharge reed valves are mounted as well as the volume of the discharge port extending through the valve plate. Since the discharge valve reed valve is on the outer side of the valve plate, and the valve plate must have a certain minimum thickness to give it the required strength, the volume of the discharge port necessarily is a major portion of the total clearance volume. The ideal compressor would have no clearance volume, and generally, the greater the clearance volume, the lower the efficiency of the compressor. The reason that clearance volume adversely affects efficiency is that this volume constitutes gases that require additional work or energy for compression on the working stroke of the piston, .and this energy is only partially recovered through reexpansion on the suction stroke as the cylinder is refilled through the suction port. Thus, reduction of the clearance volume will increase the efficiency of the compressor as long as other factors are not also adversely affected.
Since the clearance volume consists mostly of the above-described two components, efforts to reduce this volume have taken the form of minimizing the distance between the piston face and the valve plate, or more specifically, the valve sheet incorporating the suction valve reed. As for the volume of the discharge port, the diameter cannot be reduced below a certain minimum because this would increase the restriction on discharge flow, and the length of the port must be sufficient in terms of valve plate thickness for the necessary strength to resist the forces of the compressed refrigerant. While some port length reduction has been accomplished by recessing the discharge valve in the valve plate as disclosed in U.S. Pat. No. 4,723,896, granted Feb. 9, 1988 to J. F. Fritchman and assigned to the assignee of the present invention, it is still necessary to have a sufficient thickness of the valve plate material so that the discharge port remains a substantial portion of the total clearance volume.
Another approach to the matter of reducing the clearance volume of the discharge port has been to provide a plug or projection on the piston face that extends into and therefore partially fills the discharge port. This can be done as shown in the present inventor's U.S. Pat. No. 5,149,254, granted Sep. 22, 1992, but this arrangement has a relatively high cost of manufacture and still requires clearance between the sides of the plug and the wall surface of the port itself.
Still another approach has been to provide a valve which projects beyond the actual sealing valve seat into the discharge port itself to substantially fill the volume between the valve member and the piston. In the case of large, higher power compressors this has been accomplished using a centrally positioned poppet valve and a surrounding ring type suction valve. The discharge valve may take the form of either a guided poppet or as free poppet positioned by a biasing spring and a large conical valve seat with the dimensions of the poppet allowing the flat face of the poppet to extend close to the face of the piston. Examples of this arrangement are shown in U.S. Pat. Nos. 4,368,755 and 4,543,989. However, these arrangements have not been found to be applicable to the much smaller household refrigeration compressors and their limited size and simple construction.